The present invention relates to magnetic resonance imaging or "MRI".
MRI is widely used in medical and other arts to obtain images of a subject such as a medical patient. The patient's body is placed within a subject receiving space of a primary field magnet and exposed to a strong, substantially constant primary magnetic field. The atomic nuclei spin around axes aligned with the magnetic field. Powerful radio frequency "RF" signals are broadcast into the subject receiving space to excite atomic nuclei within the patient's body into a resonance state in which the spinning nuclei generate minuscule RF signals, referred to herein as magnetic resonance signals. By applying magnetic field gradients so that the magnitude of the magnetic field varies with location inside the subject-receiving space characteristics of the magnetic resonance signals from different locations within the region, such as the frequency and phase of the signals, can be made to vary in a predictable manner depending upon position within the region. Thus, the magnetic resonance signals are "spatially encoded" so that it is possible to distinguish between signals from different parts of the region. After repeating this procedure with various different gradients, it is possible to derive a map showing the intensity or other characteristics of the magnetic resonance signals versus position within the excited region. Because these characteristics vary with concentration of different chemical substances and other characteristics of the tissue within the subject's body, different tissues provide different magnetic resonance signal characteristics. When the map of the magnetic resonance signal characteristics is displayed in a visual format, such as on screen or on a printed image, the map forms a visible picture of structures within the patient's body.
MRI provides unique imaging capabilities which are not attainable in any other imaging method. For example, MRI can provide vivid, detailed images of soft tissue abnormal tissues such as tumors, and other structures which cannot be seen readily in X-ray images. Moreover, MRI operates without exposing the patient to ionizing radiation such as X-rays. For these and other reasons, MRI is widely utilized in medicine.
Some of the primary field magnets utilized heretofore have imposed severe physical constraints on the patient and on medical personnel attending to the patient during the MRI procedure. For example, conventional solenoidal primary field magnets use a series of circular super-conducting coils spaced apart from one another along an axis. These magnets provide a small, tubular subject-receiving space enclosed within the solenoids. A patient to be imaged must slide into the tubular space. The experience is highly claustrophobic. Some obese or pregnant patients often cannot fit inside the patient-receiving space. Moreover, it is essentially impossible for a physician to reach those regions of the patient disposed inside the subject receiving space.
Attempts have been made heretofore to create "open" MRI primary field magnets using ferromagnetic frames. Although these designs provide somewhat better access to the patient for diagnostic scanning, and a somewhat less claustrophobic experience for the patient, they are less than optimal for surgical intervention. For example, these designs provide limited access of physicians and surgeons to the patient. Additionally, the designs have difficulty providing a highly uniform field with pole dimensions desirable for surgery.
As described, for example, in commonly assigned U.S. Pat. No. 4,707,663, other primary field magnets utilize ferromagnetic frames to route and concentrate magnetic flux into the patient receiving space. Primary field magnets using such a ferromagnetic frame can employ permanent magnets, resistive electromagnetic coils, or super-conducting coils having a relatively low number of ampere terms while still providing a high field strength in the patient-receiving space. Moreover, such magnet assemblies can provide excellent field uniformity. Ferromagnetic frame magnets in accordance with the '663 patent also provide a less claustrophobic, more accessible subject receiving space.
As disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 07/952,810 filed Sep. 28, 1992, the disclosure of which is hereby incorporated by reference herein, a ferromagnetic magnetic frame may include a pair of plate-like pole supports spaced apart from one another and supported above one another by a set of columns. In preferred magnets according to '810 application, the frame defines a polar axis passing through the space between the plates. Preferably, ferromagnetic poles project from the pole supports adjacent the polar axis, so that the poles define a subject receiving spacing at a medial plane, midway between the plates. The columns have unique shapes such that, in preferred embodiments, the columns flare outwardly in the radial direction, away from the polar axis adjacent the medial plane. The dimensions of each column in the circumferential direction, around the polar axis desirably taper so that the circumferential dimension of each column is at a minimum in a region adjacent the medial plane. As described in further detail in the '810 application, magnets with ferromagnetic frames in accordance with preferred embodiments of the invention taught therein can provide a unique combination of accessibility and a large, aesthetically pleasing and non-claustrophobic patient-receiving space and can also provide high field strength without resort to super-conducting coils. Even higher field strengths can be provided where superconducting coils are used. Magnets according to preferred embodiments taught in the '810 application thus provide an elegant solution to the problems of claustrophobia, lack of access and limitations on field strength and uniformity posed by prior designs. Surgical operations and other medical procedures can be performed readily on a patient while the patient is disposed inside the patient-receiving space of preferred magnets according the '810 application. The ability to perform surgical operations while the patient is disposed inside the patient-receiving space allows the physician to treat the patient under direct guidance of a MRI image acquired during the procedure itself. For example, as the surgeon advances a probe into the body to treat a lesion, the surgeon can see the probe and the lesion in the MRI image.
However, even with this enhanced design, the patient still perceives the MRI procedure as involving placement of his or her body into the interior of a machine. Moreover, the physician treating the patient still perceives that he or she must stand outside of the apparatus and reach into the apparatus to gain access to the patient. Accordingly, even further improvement in primary field magnet structures for MRI apparatus would be desirable.